mlt: fixed compilation with opencv

mlt had opencv3 as buildInput but was
actually not used in the compilation because
its usage was disabled by default in mlt's cmake.
This commit activates it and supplies the appropriate opencv version,
because mlt depends now on opencv4.

pkgs/development/libraries/mlt/qt-5.nix

@@ -16,7 +16,7 @@
 , qtsvg
 , fftw
 , vid-stab
-, opencv3
+, opencv4
 , ladspa-sdk
 , gitUpdater
 , ladspaPlugins
@@ -51,7 +51,7 @@ mkDerivation rec {
     sox
     fftw
     vid-stab
-    opencv3
+    opencv4
     ladspa-sdk
     ladspaPlugins
   ];
@@ -63,6 +63,7 @@ mkDerivation rec {
   cmakeFlags = [
     # RPATH of binary /nix/store/.../bin/... contains a forbidden reference to /build/
     "-DCMAKE_SKIP_BUILD_RPATH=ON"
+    "-DMOD_OPENCV=ON"
   ];
 
   qtWrapperArgs = [